In a fluid transfer device or printer of the type disclosed in U.S. Pat. No. 3,500,436, granted on Mar. 10, 1970, R. W. Nordin, liquid ink is squirted from a nozzle under pressure and is charged and electrostatically deflected to form alphanumeric or graphical indicia on paper. In order to obtain uniform deflection responses in the electrostatic deflection field, the ink droplets must not only be charged to the appropriate voltages, but they must also be travelling at a well-defined, preferably-constant velocity. This requires that a constant pressure be applied to the fluid behind the nozzle orifice. Any variations in fluid pressure will result in variations of fluid velocity which will result in variations in deflection magnitude for the same deflection voltages. This constant pressure must be maintained preferably over the entire period during which copy is being printed on a page of paper.
Simple, constant-pressure pumps known to the art are readily available. For example, a centrifugal pump could be used, except that this would require that the fluid be exposed to the pump impeller and its numerous seals and connections with resultant risk of contamination and impurities which would tend to clog the minute orifice of the nozzle. Such nozzles are typically on the order of .0008 inch in diameter, or smaller. Also, the handling of such ink can be an exceedingly messy operation; and risk of leakage through faulty seals is unacceptable. Additionally, rapid changes up to the desired pressure and down to zero flow are desired so as to minimize messy ink dribble on "start up" and "stop" of the printer.
Pumps are well known wherein the fluid is located in a tubular container so that the container can be squeezed between a ram and an anvil. Such pumps are readily adaptable to pumping fluid at constant volume. An example of such a constant-volume pump is an artificial heart machine in which blood flows through a hose or tube. Two or more rollers squeeze the tube progressively much like the motion of manual milking of a cow. However, when trying to get constant pressure so as to obtain constant fluid velocity, difficulties are experienced with squeezing tube pumps. As the tube or container collapses, it exposes a greater and greater area of surface to the ram. This requires either complex driving mechanisms for the ram so as to accommodate this greater area with resultant greater force, or simply a lower pressure will result.
The body of the tube or container, as it is squeezed, must also be constrained from simply balloning much as an unconstrained automobile inner tube will expand locally without the constraint of the tire carcass and rim.
It is an object of the present invention to deliver a quantity of fluid under constant pressure with a minimum likelihood of contamination.
It is another object of the present invention to constrain a fluid container as it is being squeezed in order to pressurize its contents so as to prevent balloning of the walls of the container.